Within the last decade, there have been significant advances made in the application of lasers in medicine. A variety of different types of lasers are now being used in a variety of different procedures.
One such laser system is marketed by the assignee herein under the trademark Versapulse. This system includes a laser having a gain medium formed from Holmium:YAG which generates an output wavelength of 2.1 microns. The output of the laser is delivered to the treatment site via an optical fiber. The laser system has been found to be extremely useful in a variety of orthopedic applications such a knee surgery.
In many of these orthopedic applications, the tissue to be treated is immersed in a liquid medium such as saline which primarily consists of water. The 2.1 micron wavelength output of the Ho:YAG laser is highly absorbed in water. Thus, it would be expected that a large portion of the energy emitted from the fiber would be absorbed in the liquid medium before reaching the target tissue. In fact, only a portion of the laser light is absorbed in the liquid medium due to a phenomenon referred to as the "Moses effect."
One of the earliest reports of the Moses effect in association with Ho:YAG lasers appeared in an article by van Leeuwen, et. al., in Lasers in Surgery and Medicine, ("Non-contact Tissue Ablation by holmium:YSGG Laser Pulses in Blood," Vol. 11, 1991). In this article, it was reported that when the laser energy is absorbed in the liquid medium, a vapor bubble is created which expands outwardly towards the target tissue. Once the bubble reaches the target, the laser beam can pass through the vapor to the target with very little attenuation since the density of the water molecules in the steam is many orders of magnitude less than in the liquid state.
In this initial report, it was noted that the bubble formed on time scale on the order of 100 to 200 microseconds and lasted for a few hundred microseconds more before collapsing. These time frames are on the order of the pulse widths used with the solid state holmium laser. Thus, it was initially assumed that a large portion of the pulse was necessary to form the bubble and to maintain its existence before collapsing.
In a recent presentation, the same researchers reported that the formation of the vapor bubble was not dependent upon the generation of a standard, high energy, holmium laser pulse. In contrast, the researchers found that the vapor bubble can be initiated with a very small amount of laser energy. In addition, the speed at which the bubble forms is relatively independent of the duration or excess energy in the pulse. (See, "Bubble Formation During Pulsed Laser Ablation: Mechanism and Implications", SPIE Conference on Biomedical Optics, Los Angeles, van Leeuwen et. al., Jan. 18, 1993, as yet unpublished in written form)
Based on these measurements, the researchers concluded that the vapor bubble can be formed without having to vaporize all of the liquid medium in the region in front of the delivery end of the fiber. Under normal circumstances, in order to vaporize water, its temperature must first be raised from ambient temperature to the boiling point (ie. 100 degrees centigrade). Once the temperature of the water has been raised to the boiling point, the liquid can then be converted to a gas by supplying sufficient additional energy to overcome the latent heat of vaporization. This additional energy is about an order of magnitude greater than the energy necessary to raise the temperature of the water from body temperature to the boiling point.
The amount of energy be necessary to vaporize the water at the end of the fiber to create a vapor bubble is based on the specific heat of the water and the volume of the water in front of the fiber which is absorbing the light. The volume of water is defined by the cross-sectional area of the delivery end of the fiber multiplied by the absorption length of the laser wavelength in the water.
Based on their most recently recorded measurements, the prior researchers have concluded that the vapor bubble is created at an energy level significantly below that which would be needed to overcome the latent heat of vaporization for the volume of water which absorbs the laser light. Rather, it appears the energy required to form the vapor bubble roughly corresponds to the energy required to raise the temperature of the heated volume to the boiling point, which, as noted above, is an order of magnitude less than required to vaporize the volume.
It is believed by the applicant herein that the energy of the laser pulse functions to vaporize only a tiny fraction of the volume of liquid which is absorbing the laser radiation while the remainder of the volume is heated only to the boiling point. Once the tiny volume of fluid near the delivery end of the fiber has been vaporized, the bubble will begin to form and expand at a rate which is substantially independent of the duration and energy in the pulse.